EP2598474B1 - Method for continuously producing nitrobenzene - Google Patents

Description

The present invention relates to a continuous process for the preparation of nitrobenzene by nitration of benzene with nitric acid or mixtures of nitric acid and sulfuric acid to a crude nitrobenzene, washing the crude nitrobenzene by means of at least one acidic, alkaline and neutral scrubbing to obtain a prepurified nitrobenzene in which the prepurified nitrobenzene is further purified by removal of low boilers in a distillation apparatus by evaporation of the low boilers and separation of nitrobenzene from the resulting further purified nitrobenzene in one Distillation apparatus by partial evaporation of nitrobenzene, wherein pure nitrobenzene is removed from the gas to the distillation apparatus and then condensed, and wherein the unevaporated part of the further purified nitrobenzene in a Any part of the laundry is returned.

Nitrobenzene is an important intermediate in the chemical industry, which is needed in particular for the production of aniline and thus for the production of methylene diphenyl diisocyanate (MDI) and the polyurethanes based thereon.

The nitration of benzene with nitric acid to a crude nitrobenzene has been the subject of numerous publications and patent applications. The methods common today essentially correspond to the concept of adiabatic nitration of benzene by a mixture of sulfuric and nitric acid (so-called mixed acid ). Such a method was first used in US 2,256,999 and is used in today's embodiments, for example in EP 0 436 443 B1 . EP 0 771 783 B1 and US 6 562 247 B2 described. The methods with adiabatic reaction are characterized in particular by the fact that no technical measures are taken to add or remove heat to the reaction mixture.

Also, isothermal processes for nitrating benzene with mixed acid are described, such as in EP 0 156 199 B1 described.

Also, processes for the nitration of benzene are known, which manage without the use of sulfuric acid. These are for example described in US 2,739,174 or US 3,780,116 ,

In principle, gas-phase processes for the nitration of benzene with nitric acid or nitrogen oxides are also possible, but the yields that can be achieved are still low ( EP 0 078 247 B1 . EP 0 552 130 B1 ).

All of the processes have in common that the reaction product initially formed is a crude nitrobenzene which contains nitric acid as impurities and - if mixed with mixed acid - sulfuric acid, and as organic impurities dinitrobenzene and nitrated oxidation products of benzene, in particular nitrated phenols (nitrophenols). It also contains organic compounds formed from the compounds contained as impurities in the benzene used ( WO 2008/148608 A1 ). In addition, the crude nitrobenzene also contains metal salts which may be present dissolved in the acid residues or in the crude nitrobenzene ( DE 10 2007 059 513 A1 ).

Numerous investigations have been made in the past to improve the quality of the crude nitrobenzene and thus to increase the yield of benzene and nitric acid. Thanks to these developments, today's adiabatic liquid phase processes are so mature that they all succeed in producing a crude nitrobenzene which has a low by-product content, ie contains only between 100 ppm and 300 ppm of dinitrobenzene and between 1500 ppm and 2500 ppm of nitrophenols , wherein picric acid may assume a proportion of 10% to 50% of the nitrophenols.

The crude nitrobenzene has impurities as water, benzene and nitrophenols and dinitrobenzene and - if nitrated with mixed acid - sulfuric acid, which by suitable work-up procedures such. B. washing and distillation stages are separated. One possible embodiment of this workup is in EP 1 816 117 B1 where the nitrobenzene is subjected to an acidic wash, an alkaline wash, a neutral wash, and finally a distillative clean. The task is the in EP 1 816 117 B1 Described distillation only to remove water and benzene from the nitrobenzene and not to evaporate as completely as possible nitrobenzene. The "pure" nitrobenzene of such a process is therefore the bottom product of the distillation and is according to EP 1 816 117 B1 characterized in that it has a conductivity of <50 μS / cm, preferably of <25 μS / cm and particularly preferably of <10 μS / cm (method for conductivity measurement see here, paragraph [0025]). To choose the conductivity measurement as the sole measure of the nitrobenzene purity is disadvantageous, since this identifies only water-soluble and dissociable compounds. High molecular weight compounds such. B. nitrated biphenyls (see Example 1) and non-dissociable metal compounds such as iron oxides, and poorly water-soluble compounds such as calcium sulfate, silicates or metal complexes are not recorded and could thus unnoticed remain in the nitrobenzene, since the "pure" nitrobenzene in EP 1 816 117 B1 represents only the bottom product of the distillation. High-boiling organic compounds and salts can not be separated with such a distillation. In particular, salts may be used in the further use of the purified Nitrobenzene, such as in the hydrogenation to aniline (see below), cause great problems in that they lead to the formation of deposits in apparatus (eg evaporators) or, if they reach the reaction zone, a premature deactivation of the catalyst result can have.

When the nitrobenzene production literature refers to the distillation of nitrobenzene, it almost always refers to the distillative separation of water and benzene, while the nitrobenzene itself is the bottom product of such distillation in the processes known in the art. Before evaporation of the nitrobenzene itself is in some writings even for reasons of safety explicitly discouraged. So describes US 4,021,498 a specific method for adiabatic nitration of benzene, which aims to minimize the content of dinitrated compounds in the product (see column 2, lines 18 to 29). US 4,021,498 clearly points out the dangers associated with evaporation of the nitrobenzene itself (col. 2, lines 7 to 17) and thus teaches to purify the nitrobenzene by evaporation and recondensation.

Only a few pamphlets on the preparation or purification of nitrobenzene teach actually an evaporation of the nitrobenzene itself. So describes US 1,793,304 a process for the purification of (inter alia) nitrobenzene, in which contaminated nitrobenzene is first purified by filtration through so-called fuller's earth and then by heat treatment with alumina or other basic oxides at 80 ° C to 100 ° C (p 1, lines 63 to 71). The treatment with alumina is preferably followed by distillation in a partial vacuum at bottom temperatures of 140 ° C. to 160 ° C. (page 1, lines 74 to 79). Further details on the execution of the distillation does not mention the document; however, due to the boiling point of nitrobenzene at atmospheric pressure (211 ° C), it can be assumed that nitrobenzene is actually vaporized in said temperature range and in the "partial vacuum". Information on the recovery of the energy used in the distillation does not provide this document.

US 2,431,585 describes gas phase nitration of benzene with final distillation of the nitrobenzene overhead the distillation column 14 (see figure). A partial discharge of the distillation bottoms is not provided.

CH 186266 discloses a process for purifying technical grade nitrobenzene in which impurities such as dinitrobenzene are converted to other easily separable compounds prior to distilling off the nitrobenzene by chemical reaction with basic compounds and organics (see dependent claim 1, examples 2 to 6). The distillation conditions given are 122 ° C. and 66 mbar (see Example 1). At the level of these distillation temperatures can the condensation heat not make sense, z. B. in the form of usable steam, recover.

RU 2 167 145 C1 describes the separation of high-boiling compounds - in particular sulfur-containing organic compounds - from nitrobenzene by rectification of the nitrobenzene, wherein nitrobenzene is taken overhead the rectification. However, this document does not address the particular problem of salts in crude nitrobenzene, which can lead to blockage of packs and heat exchangers. Also, the in RU 2167145 C 1 chosen distillation conditions disadvantageous, since only low pressures (20 mm Hg to 80 mm Hg, corresponding to 27 mbar to 107 mbar, see claim 1) can be applied, resulting in condensation temperatures of only 99 ° C to 134 ° C, as the expert by Calculation (for example, by calculating the saturation vapor pressure curve using the Antoine equation, see Brown, Australian Journal of Scientific Research, Series B: Biological Sciences (1952), 5A, 530-540 ) can easily determine. At this level of these condensation temperatures, the heat of condensation does not make sense, for. B. in the form of usable steam, recover.

CN 100999472 A describes the workup of the bottom residue of the nitrobenzene distillation to obtain m-dinitrobenzene, without going into more detail on the conditions of the distillation of the nitrobenzene.

The purified nitrobenzene (pure nitrobenzene) is mainly used for the production of aniline, which today in turn takes place mainly by catalytic hydrogenation of the nitrobenzene in the gas phase with hydrogen. For conversion into the gas phase, nitrobenzene can either be evaporated ( EP 0 696 574 B1 Paragraph [0024]) or into a hot gas stream, preferably into a hydrogen stream ( DE-OS-1 809 711 . DE 10 2006 035 203 A1 , Paragraph [0053]). Evaporation into the stream of fresh hydrogen is considered to be advantageous since it is intended to form significantly lower deposits in the reactor and in the supply lines ( EP 0 696 574 B1 Paragraph [0024]). However, evaporation into the fresh hydrogen stream does not completely avoid the formation of deposits in the reactor since metal compounds, salts and high boilers present in the nitrobenzene will continue to accumulate there. The accumulation of solids (such as high-boiling organic compounds and salts) in the reactor is disadvantageous because it can shorten the purification intervals of the reactor and catalyst can be deactivated prematurely. If the nitrobenzene is evaporated in a conventional evaporator, while it is possible to minimize problems with deposits in the reactor, it shifts however, to the evaporator stage: Frequent required cleaning of the evaporator always lead to interruptions in the continuous operation of the aniline plant. In addition, depending on the quality of the nitrobenzene used, the above-mentioned safety problems may occur. There is therefore a need for a production method for nitrobenzene which makes this available from the outset with the necessary purity, without requiring further purification steps in process steps of aniline production.

The hydrogenation of nitrobenzene to aniline can be carried out on fixed catalyst beds in tube-bundle reactors ( DE-AS-2 201 528 ) or tray reactors ( EP 0 696 574 B1 , Paragraph [0021]) or in fluidized bed reactors ( DE-AS-1 114 820 ). Numerous publications describe catalysts for the gas-phase hydrogenation of nitroaromatics. The hydrogenation-active elements include Pd, Pt, Ru, Fe, Co, Ni, Cu, Mn, Re, Cr, Mo, V, Pb, Ti, Sn. Dy. Zn, Cd, Ba, Cu, Ag, Au. Likewise, these elements in the form of their compounds, for example as oxides, sulfides or selenides and also in the form of a Raney alloy and on supports such as Al ₂ O ₃ , Fe ₂ O ₃ / Al ₂ O ₃ , SiO ₂ , silicates, Coals, TiO ₂ , Cr ₂ O _{3 are used} as the hydrogenation catalyst.

The reactors can be isothermally (using cooling) DE-AS-2 201 528 ) or adiabatic ( EP 0 696 574 B1 ) operate. Combinations of isothermal and adiabatic reaction sections are also possible ( GB 1 452 466 ).

All publications in which the evaporation of nitrobenzene is disclosed, whether in the context of a nitrobenzene synthesis in a distillation apparatus, or in the context of the evaporation of the reactant nitrobenzene in a gas phase aniline synthesis, has in common that the most complete evaporation of the nitrobenzene is desired. This has the consequence, on the one hand, that safety problems can occur during evaporation (see above) and, on the other hand, deposits of high-boiling organic secondary components and salts in evaporators and distillation apparatuses which can severely disrupt the operation and lead to production stoppages.

It was therefore an object of the present invention to design the workup of crude nitrobenzene so that it is as trouble-free as possible (no system failure due to removal of salt deposits in a distillation column which has become necessary) (no problems due to the accumulation of potentially explosive compounds in the bottom of a Distillation column) and energy-efficient (optimum utilization of heat occurring in the process) can be carried out.

A further object of the present invention is to provide a pure nitrobenzene in which high-boiling organic compounds and salts are either completely removed or at least sufficiently depleted that this pure nitrobenzene can be used without further disadvantages in other uses.

In particular, it was also an object of the present invention to provide a pure nitrobenzene, with an aniline process with high reactor life (= long cleaning intervals) can be operated. To provide such a nitrobenzene, the quality of the nitrobenzene can not be evaluated, as described in the prior art, solely on the basis of its conductivity. Thus, it was found that a nitrobenzene whose conductivity was less than 10 μS / cm and therefore in the sense of EP 1 816 117 B1 may be considered pure, may still contain up to 200 ppb (0.2 ppm) of sodium. The use of such a nitrobenzene in a gas phase aniline process is disadvantageous, as it can lead to contamination of nitrobenzene heaters and evaporators, or in the case of atomization of the nitrobenzene to contamination of the catalyst surface, which leads to a reduction in selectivity and / or the life of the catalyst can lead (see Examples 4 and 5).

1. a) Nitrierung von Benzol mit Salpetersäure oder Gemischen aus Salpetersäure und Schwefelsäure und anschließende Abtrennung überschüssiger in der Nitrierung eingesetzter Säure, wobei Roh-Nitrobenzol erhalten wird,
2. b) Waschen des Roh-Nitrobenzols aus Schritt a) mittels mindestens jeweils einer sauren, alkalischen und neutralen Wäsche in bevorzugt dieser Reihenfolge, wobei nach Abtrennung der in der letzten Wäsche eingesetzten Waschflüssigkeit ein vorgereinigtes Nitrobenzol erhalten wird, welches neben Nitrobenzol mindestens noch Leichtsieder, ggf. Mittelsieder, sowie Hochsieder und Salze enthält,
3. c) Abtrennung von Leichtsiedern aus dem vorgereinigten Nitrobenzol aus Schritt b) in einem Destillationsapparat durch Verdampfung der Leichtsieder, wobei das Sumpfprodukt Nitrobenzol an Leichtsiedern abgereichert und auf diese Weise weiter aufgereinigt wird,
4. d) teilweise Abtrennung von Nitrobenzol aus dem weiter aufgereinigten Nitrobenzol aus Schritt c) in einem Destillationsapparat durch teilweise Verdampfung von Nitrobenzol, und zwar bevorzugt durch Verdampfung von > 80 Massen-% bis ≤ 99,9 Massen-%, bevorzugt zwischen > 90 Massen-% und ≤ 95 Massen-%, des weiter aufgereinigten Nitrobenzols, bezogen auf die Gesamtmasse des weiter aufgereinigten Nitrobenzols, wobei Rein-Nitrobenzol dem Destillationsapparat gasförmig entnommen und anschließend kondensiert wird und unverdampftes Nitrobenzol, ggf. Mittelsieder, sowie Hochsieder und Salze aus dem Sumpf des Destillationsapparates teilweise bis vollständig, bevorzugt vollständig, ausgeschleust werden, und
5. e) Rückführung des nicht verdampften Teils des weiter aufgereinigten Nitrobenzols aus Schritt d) in eine beliebige Stelle von Schritt b), bevorzugt in die saure Wäsche.

It has been found that the problem can be solved by a process for the preparation of nitrobenzene by

1. a) nitration of benzene with nitric acid or mixtures of nitric acid and sulfuric acid and subsequent separation of excess acid used in the nitration, whereby crude nitrobenzene is obtained,
2. b) washing the crude nitrobenzene from step a) by means of at least one acidic, alkaline and neutral wash in preferably this order, wherein after separation of the washing liquid used in the last washing a prepurified nitrobenzene is obtained, which in addition to nitrobenzene at least low boilers, if necessary Contains intermediate boilers, as well as high boilers and salts,
3. c) removal of low boilers from the prepurified nitrobenzene from step b) in a distillation apparatus by evaporation of the low boilers, wherein the bottoms product nitrobenzene is depleted of low boilers and further purified in this way,
4. d) partial removal of nitrobenzene from the further purified nitrobenzene from step c) in a distillation apparatus by partial evaporation of nitrobenzene, preferably by evaporation of> 80% by mass to ≤ 99.9% by mass, preferably between > 90% by mass and ≤ 95% by mass, of the further purified nitrobenzene, based on the total mass of the further purified nitrobenzene, wherein pure nitrobenzene is taken from the gas in the distillation apparatus and then condensed and unevaporated nitrobenzene, possibly medium boilers, and high boilers and salts from the bottom of the distillation apparatus partially to completely, preferably completely, be discharged, and
5. e) recycling the unevaporated portion of the further purified nitrobenzene from step d) to any desired location of step b), preferably into the acidic wash.

Excess acid used in the nitration denotes any type of unreacted acid, ie
if necessary, nitric acid when carrying out the nitration with only nitric acid
or
Sulfuric acid or optionally nitric acid and sulfuric acid when carrying out the nitration with mixtures of nitric acid and sulfuric acid.

Low boilers in the context of this invention are all compounds and azeotropically boiling mixtures of compounds whose boiling points are below that of nitrobenzene under the distillation conditions chosen. The main constituent of the low boilers is not fully reacted benzene. Further typical low-boiling components are n-hexane, cyclohexane, n-heptane, methylcyclohexane, bicycloheptane and the isomeric dimethylpentanes.

In the context of this invention, all compounds and azeotropically boiling mixtures of compounds whose boiling points are above that of nitrobenzene under the distillation conditions selected but below 350 ° C. under atmospheric pressure are referred to as medium boilers . Typical medium boilers are the isomeric dinitrobenzenes.

High boilers in the context of this invention are all compounds and azeotropically boiling mixtures of compounds whose boiling points are above atmospheric at 350.degree. The boiling points of such compounds are so high that in a catalyzed gas phase hydrogenation to aniline, which is the preferred use nitrobenzene prepared according to the invention, contamination and deactivation of the catalyst can not be excluded. Typical high boilers have more than one aromatic ring and include, for example, nitrated biphenyls, as well as nitrated hydroxy biphenyls.

Typical salts which are removed from the nitrobenzene by the process according to the invention are the sodium and calcium salts of nitrite, nitrate, sulfate and oxalate, and also silicon oxide and iron oxide.

The procedure according to the invention achieves extraordinarily high degrees of purity of the nitrobenzene. Nitrobenzene prepared by the process according to the invention has a purity with respect to organic impurities (essentially the above-mentioned medium boilers) of> 99.9500% (corresponding to <500 ppm impurities), preferably of> 99.9900% (corresponding to <100 ppm impurities) , particularly preferably greater> 99.9990% (corresponding to <10 ppm of impurities) (preferably determined by means of gas chromatography), and contains not more than 0.1 ppm, preferably not more than 0.05 ppm, particularly preferably not more than 0.01 ppm of inorganic impurities (salts , determined as cations, by atomic absorption spectrometry (Inductively Coupled Plasma, ICP) (unless otherwise stated, percentages in% and ppm always refer to the mass).

A high purity nitrobenzene prepared by the process of the invention is particularly suitable for use in the hydrogenation to aniline.

Step a) of the process according to the invention can in principle be carried out by all methods known in the prior art. Preference is given to the reaction of benzene with a mixture of nitric acid and sulfuric acid under adiabatic process control, as described in US Pat DE 10 2008 048713 A1 , Paragraph [0024] is described.

Step b) of the method according to the invention is basically known in the prior art. In this three-stage wash, the crude nitrobenzene obtained in step a) is washed with suitable washing liquids (ie wash waters of a pH corresponding to the respective stage) in three washing steps, the individual washing steps in principle being able to be carried out in any order. Preferably, however, the order is (1) acid laundry - (2) alkaline laundry - (3) neutral laundry. Particularly preferably, step b) takes place according to the method described in paragraphs [0008] to [0012] of EP 1 816 117 B1 described procedure. Particular preference is given to using electrophoresis in the neutral wash in step b), as described in US Pat EP 1 816 117 B1 , Paragraph [0013] is described.

In step c) of the process according to the invention, a rectification column is preferably used as the distillation apparatus, ie an apparatus in which at least one theoretical separation stage is realized and in which a liquid reflux is fed into the top of the column. The reflux ratio, ie the ratio of reflux to withdrawn condensate, is preferably between 0.01 and 0.5, and as reflux, one (or more) low boilers, particularly preferably benzene, is preferably used. The benzene preferably used as reflux may be fresh benzene, or more preferably benzene obtained from the top product of the rectification column after separation of the water (eg by static phase separation). Further, it is not necessary to use pure benzene in the benzene preferably used as the reflux. Rather, it may also be a mixture of substances containing benzene in addition between 0.1% by mass and 10% by mass of nitrobenzene and / or between 0.1% by mass and 50% by mass of aliphatic hydrocarbons, each based on the Total mass of the return. In step c) of the process according to the invention, nitrobenzene is the bottom product, ie only nitrobenzene from the bottom of step c) is distilled off in step d). (Eventually negligible amounts of nitrobenzene may be entrained with the low boilers in step c) and, if these do not return to the bottom, are discharged from the system.) In step d) of the present invention high boilers and salts are separated, by further subjecting the further purified nitrobenzene obtained in step c) to a distillation process in which pure nitrobenzene is taken off in gaseous form to the distillation apparatus and subsequently condensed, ie in this step the nitrobenzene itself is distilled off. Preferably, the pure nitrobenzene is removed from the distillation column overhead. However, a sidestream intake is also conceivable. The resulting in the distillation apparatus bottom product makes between> 0.1% by mass and ≤ 20 mass%, preferably between> 5 and ≤ 10 mass%, of the further purified nitrobenzene, which is introduced into the distillation apparatus in step d). The partial removal of nitrobenzene according to the invention from the further purified nitrobenzene in a distillation apparatus by partial evaporation of nitrobenzene thus means that in step d) between> 80% by mass and ≤ 99.9% by mass, preferably between> 90% by mass and ≤ 95% by mass, of the further purified nitrobenzene, based on the total mass of the further purified nitrobenzene, are evaporated.

In step e), the distillation bottoms obtained in step d) (ie the unevaporated part of the further purified nitrobenzene) are fed to any desired location of the laundry (step b). This procedure according to the invention, in which the bottom of the distillation column in step d) not completely evaporated, on the one hand increases the safety of the process. On the other hand, problems with solids separation in apparatuses are avoided because a still liquid bottoms product is discharged and returned to the laundry (step b)). There salts are washed out, and discharged with the bottom nitrobenzene is transferred to negligible residual levels in the organic phase of the respective washing stage, ie it is recycled to the prepurified nitrobenzene. As a result, yield losses are minimized.

Preferably, the pressure in the distillation step d) is chosen so that there is such a condensation temperature of the pure nitrobenzene, in which the heat of condensation can be used in an economical manner for vapor recovery. The invention accordingly also relates in particular to a process in which absolute pressures in the range between 150 mbar and 1000 mbar, preferably between 200 mbar and 600 mbar, more preferably between 400 mbar and 500 mbar, are observed in step d) and in which the in the condensation of the pure nitrobenzene in step d) heat to be dissipated to recover steam is used. This in step d) prevailing pressure is preferably measured at the condenser, in which the distillation apparatus gaseous leaving pure nitrobenzene is condensed. All commonly used in the prior art capacitors can be used for this purpose, preferred are tube bundle heat exchangers or plate heat exchangers.

At such pressures in step d) are condensation temperatures of pure nitrobenzene between 140 ° C and 210 ° C, preferably between 150 ° C and 190 ° C, more preferably between 175 ° C and 185 ° C, a. In the process according to the invention, in the condensation of the pure nitrobenzene in step d), particularly preferably in the entire step d), very particularly preferably in the entire steps a) to d), a temperature of 210 ° C. is not exceeded at any time. Only if the pure nitrobenzene is condensed at temperatures of at most 210 ° C, it can be obtained free of or at least very poor, especially organic, high-boiling compounds ("high boilers"). It has been found that form at temperatures> 210 ° C from nitrobenzene high boilers (see Example 2). Therefore, the provision of salt-free nitrobenzene, which is also free of or at least very low in high boilers, while maximizing the yield (based on benzene) surprisingly only if the maximum temperature at which the nitrobenzene is condensed (ie in general the liquid phase temperature in the condenser) does not exceed 210 ° C. Now, however, a high condensation temperature is to be striven for in principle, since only if the condensation temperature is sufficiently high can the heat of condensation be utilized in an economical manner for obtaining steam. Therefore, the actual condensation temperature to be set is i. A. a Compromise and preferably ensures an absolute pressure of the recovered vapor of at least 4 bar, which corresponds to a condensation temperature of 150 ° C.

Since the preferred use of the pure nitrobenzene according to the invention is hydrogenation to aniline, in the separation of high boilers in step d) particular attention must be paid to those compounds which are non-gaseous under the conditions of a gas phase aniline process. Preferably, the aniline production is carried out according to the method DE 10 2006 035 203 A1 , Particular preference is given to maintaining the ranges of temperature, pressure, water content in the educt gas stream and excess of hydrogen mentioned in paragraph [0018]. Nitrobenzene is preferably converted into the gas phase by means of atomization as described in Section [0053]. High-boiling organic compounds which are non-gaseous under the conditions of such a gas-phase aniline process and salts present in the prepurified nitrobenzene can be separated in step d), the nitrobenzene workup upstream of the aniline preparation preferably in a distillation apparatus without separation internals. In a distillation without separating internals is also called evaporation. Preferably, however, the distillation apparatus has demisters or mist eliminators.

Accordingly, a method is particularly preferred in which the distillation apparatus in step c) is a rectification column which is operated with partial reflux of low-boiling components, preferably benzene, more preferably benzene containing nitrobenzene and / or aliphatic hydrocarbons, and in step d) Apparatus without separating internals is. In step d) can also be dispensed with a reflux (which in this case would be part of the condensed pure nitrobenzene). In this embodiment, the invention accordingly relates to a process in which the distillation apparatus in step c) a rectification column, which is operated with partial reflux of low boilers, preferably benzene, more preferably of benzene containing nitrobenzene and / or aliphatic hydrocarbons, and in step d ) an apparatus without separation internals, which is operated without reflux condensed pure nitrobenzene is. This particularly preferred embodiment of the two-stage (steps c) and d)) distillation according to the invention ensures on the one hand that the low boilers are efficiently separated from the nitrobenzene (rectification in step c)) and, on the other hand, that the separation of high boilers and especially salts in Step d) takes place without the risk of clogging of packs in the distillation column. Possibly. in step d) not completely separated middle boilers are in pure nitrobenzene only in uncritical contents of <500 ppm, preferably <100 ppm, more preferably <10 ppm, based on the mass of pure nitrobenzene included.

Since the formation of high-boiling organic compounds (such as the said nitrated biphenyls) can also take place at temperatures lower than 210 ° C, when the nitrobenzene is exposed to these temperatures for too long , it is advantageous if the apparatus used have a short residence time. For example, self-circulating evaporators, eg. B. so-called. Robert evaporator, as an evaporator for the distillation columns in steps c) and d) are used (see Reinhard Billet: Evaporation and its Technical Applications, Weinheim, 1981, p. 119 f. In such evaporators, the residence time of the nitrobenzene is preferably 0.1 minutes to 120 minutes, more preferably 0.1 minutes to 20 minutes.

The separation of low and high boiling compounds and salts from the nitrobenzene can be combined in a dividing wall column. (The general operating principle of dividing wall columns, for example, in G. Kaibel, "Distillation Columns with Vertical Partitions", Chem. Eng. Technol. 1987, 10, 92-98 and G. Kaibel, "Industrial use of dividing wall columns and thermally coupled distillation columns", Chemie Ingenieur Technik 2003, 75, 1165-1166 Accordingly, in this embodiment, the invention particularly relates to a process in which the same distillation apparatus is used in steps c) and d) and this is a dividing wall column.

Examples Example 1 (according to the invention): separation of high boilers by distillation of the nitrobenzene - step d)

277.6 g of a nitrobenzene having no detectable amounts of low boilers and 55 ppm by area of high boilers (determined by gas chromatography, GC) were distilled in a distillation apparatus consisting of glass flask and distillation bridge at 180 ° C. and 400 mbar (absolute) ( Step d) of the process according to the invention), so that 271.4 g of distillate and 6.2 g were obtained as the bottom product. About 3700 area ppm of high boilers were found by GC in the distillation bottoms. In the group of high boilers 2-nitrobiphenyl and 2,2-dinitrobiphenyl could be identified by GC-MS (mass spectrometry). In the distillate (= pure nitrobenzene) no high boilers could be found, which shows that with the inventive method, a high-boiling nitrobenzene at low distillation temperatures (<210 ° C) can be obtained.

Example 2 (Comparative Example): Formation of high boilers by tempering

In a stainless steel autoclave nitrobenzene was a purity (GC)> 99.9900% and with a salt content of <0.1 ppm (which can be referred to as free from low boilers, high boilers and salts in the present application), for a period of 2 hours exposed to elevated temperature. After cooling, the nitrobenzene was analyzed by gas chromatography for the content of high-boiling impurities. The results are shown in Table 1 and show that high-boiling compounds have been formed solely because of the thermal load, and thus that nitrobenzene can not be exposed to arbitrarily high temperatures, if it is to be obtained high-boiling. <b><u> Table 1 </ u></b> Temperature of annealing [° C] 240 260 280 Annealing time [h] 2 2 2 Proportion of high-boilers [area ppm] 193 303 589

Examples 3 to 5 for the use of nitrobenzene of different salt content in the hydrogenation to aniline (not according to the invention)

The test system used for the example reactions is a 500 mm long reaction tube made of stainless steel. Through this reactor, a circulating gas stream is passed, which is heated by means of a heat exchanger to 250 ° C. By means of metering pumps nitrobenzene is conveyed to a nozzle and finely atomized in the circulating gas stream, where it then evaporates. Hydrogen is preheated in a heat exchanger and added to the cycle gas in front of this nozzle. The hydrogen supply is controlled by a mass flow controller. The load of the catalyst in the reaction tube was set in all examples to a value of 1.0 g _{nitroaromatic} / (ml of _catalyst · h), and the hydrogen: nitrobenzene ratio in the reactor was set to about 80: 1.

A 400 mm high bed of catalyst is placed on a sieve inside the reaction tube. After exiting the reactor, the reaction product is cooled with water. The non-volatile constituents are condensed out and separated from the gaseous components in a downstream separator. The liquid components are led out of the separator into the product collection container and collected there (glass container). In front of the collection container there is a sampling point where samples of the product can be taken at regular intervals. These are analyzed by gas chromatography. The service life of the catalyst corresponds to the time from the beginning of the reaction until complete conversion of the nitrobenzene is no longer achieved and at the sampling point> 0.1% of nitrobenzene in the product are detected by gas chromatography.

In Examples 3 to 5, in each case a nitrobenzene was used which contains about 250 ppm of organic impurities and thus fulfills the requirements according to the invention with regard to the content of organic impurities (<500 ppm). However, the nitrobenzene grades in Examples 3 to 5 differ in their salt content (exemplified by the content of sodium ions).

The examples were carried out with the catalyst system 9 g / l _carrier Pd, 9 g / l _carrier V, 3 g / l _carrier Pb on α-alumina (see EP 0 011 090 A1 ). Different aged and pretreated catalysts of this catalyst system were used; the reaction tests were stopped each time the service life of the catalyst was reached.

Example 3 (not according to the invention): Use of nitrobenzene with a content of sodium ions of <0.1 ppm

Fresh catalyst was placed in the reaction tube and purged first with nitrogen, then with hydrogen. Subsequently, the catalyst was exposed for a period of 48 h at 240 ° C with 1000 l / h of hydrogen. Then, evaporated nitrobenzene was passed to the catalyst. The nitrobenzene load was slowly increased to the desired value of 1 g _{nitroaromatic} / (ml _catalyst · hr) so that the temperature in the reactor did not rise above 450 ° C and the hydrogen addition was adjusted so that the molar ratio of hydrogen: nitrobenzene 80: 1 was. As soon as no more complete conversion of nitrobenzene took place (more than 0.1% of nitrobenzene in the reaction product), the reactant feed was stopped and the reactor was rendered inert with nitrogen. At 270 ° C., coking was then burned off in the air stream until less than 0.2% by volume of CO _{2 could be} detected in the exhaust gas. This cycle of reaction and regeneration of the catalyst was carried out a total of 3 times (Examples 3a to 3c). The service lives were respectively 983 h, 964 h and 968 h.

Example 4 (Comparative Example): Use of nitrobenzene with a content of sodium ions of 0.2 ppm

Fresh catalyst was placed in the reaction tube and purged first with nitrogen, then with hydrogen. Subsequently, the catalyst was exposed for a period of 48 h at 240 ° C with 1000 l / h of hydrogen. Then, evaporated nitrobenzene was passed to the catalyst. The nitrobenzene load was slowly increased to the desired value of 1.0 g _{nitroaromatic} / (ml _catalyst · hr) so that the temperature in the reactor did not rise above 450 ° C and the hydrogen addition was adjusted so that the molar ratio of hydrogen: nitrobenzene 80: 1. As soon as no more complete conversion of nitrobenzene took place (more than 0.1% of nitrobenzene in the reaction product), the reactant feed was stopped and the reactor was rendered inert with nitrogen. At 270 ° C., coking was then burned off in the air stream until less than 0.2% by volume of CO _{2 could be} detected in the exhaust gas. This cycle of reaction and regeneration of the catalyst was carried out a total of 3 times (Examples 4a to 4c). The service lives were respectively 953 h, 848 h and 736 h and were thus - especially in the 2. and 3rd cycle - significantly worse than when using a nitrobenzene according to the invention.

Example 5 (Comparative Example): Use of nitrobenzene with a content of sodium ions of 0.4 ppm

Fresh catalyst was placed in the reaction tube and purged first with nitrogen, then with hydrogen. Subsequently, the catalyst was exposed for a period of 48 h at 240 ° C with 1000 l / h of hydrogen. Then, evaporated nitrobenzene was passed to the catalyst. The nitrobenzene load was slowly increased to the desired value of 1.0 g _{nitroaromatic} / (ml _catalyst · hr) so that the temperature in the reactor did not rise above 450 ° C and the hydrogen addition was adjusted so that the molar ratio of hydrogen: nitrobenzene 80: 1. As soon as no more complete conversion of nitrobenzene took place (more than 0.1% of nitrobenzene in the reaction product), the reactant feed was stopped and the reactor was rendered inert with nitrogen. At 270 ° C., coking was then burned off in the air stream until less than 0.2% by volume of CO _{2 could be} detected in the exhaust gas. This cycle of reaction and regeneration of the catalyst was carried out a total of 3 times (Examples 5a to 5c). The service lives were respectively 946 h, 768 h and 605 h and were thus even worse than in Example 4.

The following table again compares the results of Examples 3 to 4: <b><u> Table 2 </ u></b> not according to the invention comparison example Lifetime [h] example Lifetime [h] example Lifetime [h] 3a 983 4a 953 5a 946 3b 964 4b 848 5b 768 3c 968 4c 736 5c 605

Example 6 (Inventive): Energy Recovery in Condensation

By means of an Aspen simulation, the mass and energy balance for the additional purification step d) for the nitrobenzene was calculated by means of evaporation and condensation. A continuous evaporator is fed with 50 t / h of nitrobenzene at a temperature of 183 ° C. At an absolute system pressure of 500 mbar, 90% of the feed (i.e., 90% by mass of further purified nitrobenzene) is vaporized using 20 bar steam. The remaining 10% are removed from the evaporator and returned to the acid wash. The energy required for evaporation is 4.7 MW or 8.6 t / h 20 bar steam.

The vapors are condensed in a downstream heat exchanger, which is operated on the process side at 490 mbar absolute. For cooling condensate is used with 100 ° C flow temperature, which is used to generate 6 bar steam. In the condensation step, an energy of 4.7 MW is released. Since the condensate must still be preheated from 100 ° C to the evaporation temperature of 159 ° C, 7.2 t / h 6 bar steam generated in this apparatus.

By using the thermal energy accumulating in the condenser, the energy required for the evaporation of the nitrobenzene can be completely recovered. The pressure on the process side is chosen so that the MNB can be vaporized with the higher of the available pressure levels of the steam and in the condenser steam of the lower of the available pressure levels can be generated.

Claims (6)

1. Process for the production of nitrobenzene by

   a) nitration of benzene with nitric acid or mixtures of nitric acid and sulfuric acid and subsequent separation of excess acid used in the nitration, wherein crude nitrobenzene is obtained,

   b) washing of the crude nitrobenzene from step a) by means of at least one of each of an acid, alkaline and neutral washing, wherein there is obtained after separation of the wash fluid used in the last washing a pre-purified nitrobenzene which, in addition to containing nitrobenzene, also contains at least low boilers,

   c) separation of low boilers from the pre-purified nitrobenzene from step b) in a distillation apparatus by evaporation of the low boilers, wherein the bottom product nitrobenzene is depleted of low boilers and thus further purified,

   d) partial separation of nitrobenzene from the further purified nitrobenzene from step c) in a distillation apparatus by partial evaporation of nitrobenzene, wherein pure nitrobenzene is removed from the distillation apparatus in gaseous form and is subsequently condensed, and

   e) return of the non-evaporated portion of the further purified nitrobenzene from step d) into any desired point of step b).
2. Process according to claim 1, in which absolute pressures in the range from 150 mbar to 1000 mbar are maintained in step d) and in which the heat to be dissipated in the condensation of the pure nitrobenzene in step d) is used to generate steam.
3. Process according to claim 1 or 2, in which the distillation apparatus
   in step c) is a rectification column which is operated with partial reflux of low boilers, and
   in step d) is an apparatus without separation-active internals.
4. Process according to claim 3, in which the distillation apparatus in step d) is operated without reflux of condensed pure nitrobenzene.
5. Process according to claim 1, in which the same distillation apparatus is used in steps c) and d) and the distillation apparatus is a dividing wall column.
6. Process according to claim 1, in which an electrophoresis is used in the neutral washing in step b).